Main extracellular electrolyte controlling water balance and nerve/muscle excitability.
What it is
Serum sodium in mmol/L (Pathology Harmony adult band). It is about water balance relative to sodium, not just “salt in the diet”.
Serum sodium is reported in mmol/L and primarily reflects water balance relative to sodium stores. Pathology Harmony adult bands are widely used in UK labs. Spot urine sodium and osmolality tests are added when hyponatraemia is investigated in hospital.
Why it matters
Hyponatraemia has many causes: medicines, illness, hormone issues, and, in endurance sport: over-drinking hypotonic fluids. Hypernatraemia often reflects water loss. Significant shifts need medical assessment; do not self-correct with salt loading.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
Sodium gradients drive action potentials in nerves and muscle and determine extracellular tonicity. Brain cells are sensitive to rapid sodium/water shifts: why correction is done carefully in hospital.
For athletes
Exercise-associated hyponatraemia is a known risk in ultras when athletes over-drink. Thirst-guided drinking and race medical plans beat aggressive salt tablets without a protocol. Dizzy, confused, or swollen hands mid-race, seek race medical help.
